Conventional threaded fixing devices such as screws are difficult to secure in masonry substrates since it is difficult for a conventional thread to find secure location within a bore in such a substrate. Conventional screw threaded fixings are accordingly conventionally secured within bores in masonry substrates by first lining the bore with a lining of relatively soft material into which the threaded fixing can cut its own thread, at the same time compressing the lining against the walls of the bore within the masonry substrate. A typical example of such a lining is that sold under the trade mark Rawlplug. Such linings are available in fibrous and plastics material form and in a wide variety of configurations reflecting a very considerable activity in the art over the years to improve upon the security and ease of use of screw threaded fixing devices used with such liners.
Adopting a somewhat similar principle, alternative forms of fixing device are of metallic material and structured so as to be expansible after introduction into a bore in a masonry material whereby compressive forces against or impingement into the internal surfaces of the bore resist withdrawal of the fixing device from the bore. Reflecting similarly substantial activity in the art, a wide variety of such devices are available. For example, various devices of this kind are available under the above-mentioned trade mark Rawlplug and under the trade mark Fischer.
The Upat TOP self undercutting anchor comprises an internally threaded cylindrical tubular steel sleeve and a cone received in one open end of the sleeve. A splined lower part of the cylindrical sleeve is in contact with the inclined walls of the cone, these wall being divergent in the direction away from the sleeve. The above described assembly is inserted into a pre-drilled bore until the extremity of the cone (i.e. the base of the conical portion) comes into contact with the end of the bore. Application of light pressure from a hammer drill causes the splined section to be forced over the conical surface, the splines having tungsten carbide tips which undercut the substrate material. The Upat EXA Express Anchor also operates an expansion fixing but does not operate on the undercutting principle.
Fixing devices somewhat similar to those sold under the trade mark Rawlplug are also sold under the trade mark Fischer, as mentioned earlier. Fixing devices are also made by Fischer which are somewhat similar in structure and operation to the above-described Upat fixing elements. The Fischer Wallbolt comprises a segmented malleable iron expander shield forming a sleeve about an orthodox screw threaded bolt. The combined assembly is in use inserted into a pre-drilled bore in a masonry substrate and the bolt is tightened with a spanner. This draws a terminal conical wedge backwards into the shield and forces four shield segments outwardly against the sides of the bore.
All fixing elements which rely on compression of an associated member into the wall of a pre-drilled bore give rise to structural disadvantages. The wall of the bore are placed under stress and in many substrate materials there is insufficient strength in the substrate structure to prevent cracking. This is particularly the case with such materials as lightweight autoclaved aerated concrete but also applies to brick structures. It is especially evident adjacent apertures in a masonry structure where on one side at least there will be comparatively small thickness of substrate material. Of course, fixing elements are commonly required in aperture regions for the fixing of window frames, door liners and similar fittings.
Equally, a good deal of activity has been focused on improvement of screw threaded members themselves in order that they should find secure fixing in masonry materials without the use of liners and without internal expansion. The driving force in these respects is, of course, the fact that liners do not provide totally satisfactory degrees of security, the fact that the need to insert a liner in a bore represents an additional activity which has associated labour costs and the fact that in the case of expansion bolts and such like the compressive forces necessary for security of fixing can result, as noted above, in rupture of masonry materials particularly when adjacent to extremities of bodies of such materials.
UK Patent Specifications Nos 2 115 511 (ITW III) and 2 152 171 (ITW II) disclose masonry fixing comprising a shank having spaced threads in the form of a helical ridge alternating with a parallel land. The flanks of the threads intersect at an angle of 50.degree. to 65.degree. and exhibit a helix angle of 6.degree. to 8.5.degree.. These features are necessarily combined with special conditions for introduction into a masonry bore and are claimed to provide security of fixing without thread striping upon insertion. Both ITW II and III represent examples of effort in the art to provide threaded devices for direct securement in a bore so as to avoid the disadvantages of expansion-type devices enumerated above.
Coarse deep threads are also disclosed in a fixing for use in masonry-like structures in UP Patent Specification No 1 510 686 (ITW IV), a further example of an attempt to provide a direct-fixture fixing agent. In this particular arrangement, the crests of the deep threads are notched in order to assist in the cutting of a thread by the fixture in use and to assist in the transmission or masonry debris within the bore. An intermediate shallower thread ridge appears to be the key to providing a design which enables security of fixture to be achieved whilst at the same time recognizing the need to minimize disturbment of the substrate material. Nevertheless, threads penetrate the substrate at fairly closely spaced positions such that pull-out performance is impaired by the shear resistance of the relatively small sections of substrate material between helical ridge turns. A similar arrangement is disclosed in European Patent Application No 0 133 773 (ITW I).
UK Patent Specification No 466039 (Jasper) discloses a fastening screw having a shank provided with a helical thread groove alternating with a land between the grooves. A longitudinal slop provides self-tapping capacity for the course deep thread. Jasper does not disclose the fastening screw for use in masonry structures.
A somewhat similar screw device is described in UK Patent Specification No 274833 (Rosenberg). The entry end screw thread configuration has a groove which penetrates the normal diameter of the fixing device together with raised border ridges, this configuration extending over a short length only of the shank. Rosenberg does not recommend the fixing device for masonry fixing, referring generally to fibrous and non-flowing materials and in particular to European Patent Specification No 0 225 003 (JSM) and UK Patent Application No 2036227 (Tomoyasu) disclose self-locking screws having a somewhat similar configuration. Helical ridges either side of a small land have their crests grooved. Tomoyasu also refers to a structure in which the ridge crests are not grooved but an intermediate ridge is turned through the land so that there are in fact two parallel grooves forming an overall thread. Both JSM and Tomoyasu relate to self-locking devices reliant on the interaction of male and female threads and there is no disclosure of a use in which the male threaded portion is self-tappingly threadedly into a masonry bore.
UK Patent Specification No. 2042120 (Gutshall) discloses a headed screw which incorporates a long-pitch helical thread rolled on a cylindrical blank. The rolling leaves untouched a land between adjacent convolutions of the thread. At each juncture between the root of the thread and the adjacent land a helical crest functioning as an auxiliary thread is provided to improve the holding power of the screw. Gutshall refers to use of the disclosed screw in soft, fibrous or low-density materials which are exemplified as wood and plastics. Gutshall does not appear to recommend the disclosed headed screw for fixture self-tappingly in a masonry substrate.
UK Patent Specification No 1541237 (Oratronics) discloses a threaded device for use as an endodontic stabilizer. The shank of the disclosed device is turned with a helical groove having a sufficiently wide pitch to provide a parallel inter-turn land. The crest of the ridge is grooved. UK Patent Specification No 1519139 (Crock) discloses a simpler structure for use in surgical techniques. The structure disclosed comprises a threaded shank comprising a thread groove somewhat reminiscent of that shown by Rosenberg as mentioned above. Crock refers skirtingly and somewhat speculatively to application in building construction and associated fields but there is no reference specifically to the use of the disclosed device for self-tappingly securing in a masonry bore.